linux-kernel - Re: [PATCH net] net: systemport: set dev->max_mtu to UMAC_MAX_MTU

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Date:   Fri, 18 Dec 2020 12:54:33 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev@...r.kernel.org, andrew@...n.ch,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Murali Krishna Policharla <murali.policharla@...adcom.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        "open list:BROADCOM SYSTEMPORT ETHERNET DRIVER" 
        <bcm-kernel-feedback-list@...adcom.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] net: systemport: set dev->max_mtu to
 UMAC_MAX_MTU_SIZE

On 12/18/20 12:52 PM, Vladimir Oltean wrote:
> On Fri, Dec 18, 2020 at 12:30:20PM -0800, Florian Fainelli wrote:
>> On 12/18/20 12:24 PM, Vladimir Oltean wrote:
>>> Hi Florian,
>>>
>>> On Fri, Dec 18, 2020 at 09:38:43AM -0800, Florian Fainelli wrote:
>>>> The driver is already allocating receive buffers of 2KiB and the
>>>> Ethernet MAC is configured to accept frames up to UMAC_MAX_MTU_SIZE.
>>>>
>>>> Fixes: bfcb813203e6 ("net: dsa: configure the MTU for switch ports")
>>>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>>>> ---
>>>>  drivers/net/ethernet/broadcom/bcmsysport.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/net/ethernet/broadcom/bcmsysport.c b/drivers/net/ethernet/broadcom/bcmsysport.c
>>>> index 0fdd19d99d99..b1ae9eb8f247 100644
>>>> --- a/drivers/net/ethernet/broadcom/bcmsysport.c
>>>> +++ b/drivers/net/ethernet/broadcom/bcmsysport.c
>>>> @@ -2577,6 +2577,7 @@ static int bcm_sysport_probe(struct platform_device *pdev)
>>>>  			 NETIF_F_HW_VLAN_CTAG_TX;
>>>>  	dev->hw_features |= dev->features;
>>>>  	dev->vlan_features |= dev->features;
>>>> +	dev->max_mtu = UMAC_MAX_MTU_SIZE;
>>>>  
>>>>  	/* Request the WOL interrupt and advertise suspend if available */
>>>>  	priv->wol_irq_disabled = 1;
>>>> -- 
>>>> 2.25.1
>>>>
>>>
>>> Do you want to treat the SYSTEMPORT Lite differently?
>>>
>>> 	/* Set maximum frame length */
>>> 	if (!priv->is_lite)
>>> 		umac_writel(priv, UMAC_MAX_MTU_SIZE, UMAC_MAX_FRAME_LEN);
>>> 	else
>>> 		gib_set_pad_extension(priv);
>>
>> SYSTEMPORT Lite does not actually validate the frame length, so setting
>> a maximum number to the buffer size we allocate could work, but I don't
>> see a reason to differentiate the two types of MACs here.
> 
> And if the Lite doesn't validate the frame length, then shouldn't it
> report a max_mtu equal to the max_mtu of the attached DSA switch, plus
> the Broadcom tag length? Doesn't the b53 driver support jumbo frames?

And how would I do that without create a horrible layering violation in
either the systemport driver or DSA? Yes the b53 driver supports jumbo
frames.
-- 
Florian